This invention relates to a lubricating paint with excellent formability and corrosion resistance (antirust effect).
Cold-rolled steel plates, zinc-base plated steel and aluminum plates are commonly used by manufacturers of household electric appliances, automobiles, building materials, etc. In manufacture of these products, said plates are usually subjected to paint coating and assembling after forming work such as press forming. In the forming work, a lubricant, typically press oil, is applied to the plate to afford the desired press formability since the plate itself has no satisfactory formability. So, when coating and assembling are conducted after that, it is required to remove the applied lubricant in the cleaning stage, which is indispensable and burdensome.
Recently, steel plates not necessitating use of press oil in the step of press forming by use of stock plate having a wax type lubricant applied to its surface have been manufactured for the purposes of saving the process stages, reducing the cost and improving the working environment. These steel plates, however, involve the problem that the applied lubricant needs to be removed at the subsequent coating and assembling process. Also, the environment in which press forming is conducted on stock plate applied with a wax type lubricant is not quite satisfactory although improved over the one where press oil was used. Efforts have been made for overcoming these problems and a functional surface treated plate having more appropriate lubricating surface has been developed. The surface of this plate is coated with a thin film of a composition principally made up of an organic resin and having lubricating properties. This surface-treated plate, therefore, can be subjected to press forming with no need of applying a lubricant such as press oil, and further it does not necessitate cleaning and paint base treatments after press forming.
The prior techniques relating to such functional surface treated plates are disclosed in the following patent applications: (1) JPN Pat. Pub. No. (S)63-25032, (2) JPN Pat. Appln. Kokai No. (S)62-289274, (3) JPN Pat. Appln. Kokai Nos. (S)61-227178, (S)61-227179, (S)61-231177, (S)61-279687 and (S)62-33781, (4) JPN Pat. Appln. Kokai No. (S)60-103185, (5) JPN Pat. Appln. Kokai No. (S)63-83172, and (6) JPN Pat. Appln. Kokai No. (H)2-124997. A summary account of each of these prior techniques is given below.
(1) relates to an aqueous composition for forming lubricative coating film comprising principally an organic-inorganic composite reaction product composed of a water-soluble or water-dispersible organic resin, an alkoxysilane compound and silica, and a lubricant. The film of the organic-inorganic composite reaction product is poor in flexibility, so that although containing a lubricating component, the film is unable to follow up high-speed press forming and can not provide satisfactory lubricity. (2) discloses a coating film mainly composed of a composite or mixed material consisting of an urethane resin and silicon dioxide. The film of this type of composition is unable to attain high lubricity aimed at in the present invention. In (3), an inorganic solid lubricant of graphite or molybdenum disulfide or a mixture thereof with lubricating oil is specified as lubricant, but this lubricant provides a dynamic friction coefficient of friction of greater than 0.1 and is unsuited for high grade press forming work. (4) discloses a two-layer chromate treated steel plate. Flexibility of the base resin, an urethane-modified bisphenol A type epoxy resin, is greatly improved by urethane modification, but the coating is still unsatisfactory, and although containing a lubricating component, it is unable to follow up high-speed press forming and can not provide satisfactory lubricity. (5) discloses a composition obtained by incorporating a lubricating material in a resin composition comprising an organic resin selected from epoxy resins, polyester resins and acrylic resins and a curing agent. Formability attainable by use of this resin composition is low and insufficient for high-grade press forming work schemed in the present invention. The coating provided according to (6) is good in formability but inferior to the present invention in corrosion resistance. Corrosion resistance is improved by the double layer treatment comprising application of a resin for an undercoat, but the treating process is complicated, and it is impossible to attain the objective corrosion resistance and lubricity and is different from using a single surface treating agent as in the present invention.
As seen from the above review of the prior art, there is yet available no functional surface treated plate which can satisfy both requirements for high-level formability and corrosion resistance. Further, for the improvement of working environment for coating by use of a lubricating paint, there has been a strong demand for an aqueous alternative in place of a solvent-based treating agent. The present invention, which was attained as a result of strenuous research on compositions of lubricant paints for solving said prior art problems, provides a lubricating paint for functional surface treated plates having high-press formability, that is, having excellent lubricity as well as excellent corrosion resistance, weldability and chemical resistance.